1. Field of the Invention
The present invention relates to a stencil printer or similar printer and an ink viscosity sensing device therefor.
2. Discussion of the Background
A thermosensitive digital stencil printer is extensively used today as a simple and convenient printer. The stencil printer uses a stencil having a laminate structure comprising of a thermoplastic resin film and a porous substrate formed of Japanese paper fibers or synthetic fibers or a mixture thereof. The surface of the stencil where the film is present is brought into contact with the heating elements of a thermal head or master making means. At the same time, the thermal head is energized in the main scanning direction in accordance with image data output from a document reading section. As a result, the film surface of the stencil is selectively perforated by heat to form an image in the form of a perforation pattern. While the perforated part of the stencil, i.e., a master, is conveyed by a platen roller and other rollers in the subscanning direction, the master is automatically wrapped around a print drum which is a porous hollow cylinder. Then, ink feeding means disposed in the print drum feeds ink while a press roller or similar pressing means presses a sheet against the drum via the master continuously. As a result, the ink oozes out via a porous portion included in the drum and the perforations of the master, printing an image on the sheet. This kind of printer is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 5-229243.
The ink for use in the stencil printer is, in many cases, implemented by emulsion ink containing a pigment, resin, solvent, surfactant, water, and so forth. As to viscosity, emulsion ink greatly depends on the environment due to temperature and humidity during operation, evaporation of water ascribable to a long down time, and so forth, as generally accepted. The viscosity of ink is one of critical factors which effect the amount of ink transfer to a sheet. Specifically, the viscosity of ink has critical influence on the quality of printed images, e.g., image density, evenness of a solid image, and offset.
To compensate for the environment dependency of the ink viscosity and thereby image quality, the pressure of the press roller acting on the print drum (print pressure hereinafter) may be varied, as taught in, e.g., Japanese Patent Laid-Open Publication Nos. 2-151473, 6-155880, 6-155881, 6-155882, and 6-199028. Schemes taught in these documents each use a control pattern table listing the variations of image quality in relation to various parameters including print speed, temperature, and down time. The actual print speed, temperature, down time and so forth sensed by various sensors arranged in the printer are compared with the control pattern table. Then, the print pressure is adequately controlled to correct the variation of image quality ascribable to the above parameters, insuring stable image quality at all times.
However, none of the above prior art schemes senses, or measures, the viscosity of ink directly effecting image quality or feeds it back as viscosity data. The prior art schemes simply use the ambient temperature, down time and other indirect data as so-to-speak substitute characteristic values for the viscosity of ink. Therefore, the print pressure cannot be delicately set in matching relation to actual conditions which differ from one machine to another machine. Moreover, the actual behavior of ink in the printer is apt to deviate from the data listed in the control pattern table. This kind o f deviation cannot be coped with and prevents a desired effect from being achieved.
The viscosity after a long down time, among others, is effected even by humidity. The conventional schemes described above cannot deal with this problem alone, i.e., they need humidity sensing means and other additional implementations which would further complicate the control.
The control pattern table cannot be prepared unless t h e variation of image quality is determined by experiments with respect to some different parameters including print speed, temperature, down time, and the kind of ink. Such an amount of data is difficult to deal with. In addition, a plurality of sensors are necessary for determining the variations of the individual parameters. Consequently, the control itself is unavoidably complicated and constitutes a heavy load on the development and design stage.
Conventional technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 64-18682, 5-201115, 6-340162, 7-17013, and 3-13344.